Measurement of the radiation from cobalt-60 and cesium-137 sources is the main purpose of the apparatus described here. Precision (±0.1%) and long-term stability (±0.2%) combined with portability and ruggedness were aimed for in its design. A fully guarded cavity ionization chamber is used in a Townsend balance circuit, with an amplifier-electrometer to sense the null. The ionization current is collected on a teflon-dielectric capacitor and the balancing potential, derived from a source stabilized by zener diodes, is read directly from the dial of a ten-turn potentiometer. Possible errors arising from the response of the chamber to the scattered radiation from large cobalt-60 sources are considered in the light of recently published spectral information.